Copy constructor, constness and rvalues

Question

I'm currently learning about move/copy constructors, constness and rvalues.

"Older literature" about C++ says:

"A copy constructor must take a const argument since it should be able to take temporaries/rvalues which are not modifiable"

But nowadays, with C++ 11's move constructors, we can directly handle rvalues. This brings me to the conclusion, that it's no longer mandatory to make a copy constructor take a const reference.

Is this true? (I could neither confirm nor falsify the statement with currently existing literature and community articles.) A short discussion about the pro and contras of having the copy constructor const would also be appreciated.

Answer 1

No, it's not true. You want your copy constructor to take a const T& and your move constructor to take a T&& . Why? Well think about what happens if we pass a const rvalue. It can't bind to the T&& , because it's not const . It can, however, bind to the const T& , which is able to bind to anything. If it were just a T& instead, it wouldn't be able to, and we'd never be able to copy from const rvalues.

To think about it in another way: the only time a move constructor is useful is if we have an rvalue (likely a temporary object) that we can steal from, which means that it must be non- const . All the other cases must invoke the copy constructor instead. The only reference type that will bind to everything else is a const lvalue reference.